In shoes particularly intended for mountain sports, e.g., cross-country skiing, Nordic hiking, and mountain hiking in general, attempts have always been made to produce soles making it possible to obtain, simultaneously, torsional stiffness properties in relation to the longitudinal axis of the sole combined with good flectional properties, in particular in the area of the metatarsals.
This goal is sought most notably in cross-country skiing and Nordic hiking, where the boot cooperates with the ski and must provide optimal guidance of the latter.
Furthermore, longitudinal flexibility of the sole of the boot is indispensable so as to allow smooth extension of the foot not only during actual cross-country skiing or Nordic hiking, but also when the skis are removed, to permit walking freely.
The situation is the same for boots designed specifically for walking.
Moreover, and precisely in the case of walking shoes or ski boots designed to be used for walking, even of only occasionally, attempts have been made to produce soles having the aforementioned properties, but which can also provide a certain level of comfort and which damp the points of impact produced by contact of the boot with the ground, while restoring energy.
French Patent No. 2 520 886 relates to an athletic shoe comprising a laminated sole composed of a first layer, or contact sole, whose upper part receives a second, or inserted flexible layer, in a central plane of which a reinforcement element is inserted in the area of the heel-piece and extending toward the arch.
In this case, the efficiency of the second, comfort layer is impaired because of the presence of the reinforcement element in its central plane.
Moreover, the presence of a lower portion of the insert positioned between the reinforcement element and the contact sole increases proportionally the height and weight of the assembly.
In addition, currently-marketed mountain boots include a model sold under the “HANWAG” label, which offers a sole composed of an outer contact sole on which are superposed, in succession, a shock-absorbing layer, then a very rigid assembly insole fitted with an anti-torsion insert.
Another boot, currently marketed under the tradename “ASOLO” differs basically in that the shock-absorption layer is confined to the heel area and is housed in a corresponding recess provided in the outer contact layer.
In these latter cases, the principal disadvantage lies in the fact that the rigid layer is in direct contact with the foot and imparts excessive flectional rigidity to the sole. Furthermore, shock waves are felt in more pronounced fashion by the foot.
In French Patent No. 2 556 569, applicants have also proposed a solution consisting of producing an outer sole by using at least two plastic materials having different mechanical properties and by applying the duplicate-molding technique.
In fact, this sole comprises a stiffening element, or shank, made of a rigid plastic material and elastically deformable, which incorporates an area of flection, at least in the metatarsal-phalangeal area, in the form of spaced transverse plates separated by bridges having a lesser thickness and on which a flexible plastic, elastically deformable material is added through a duplicate-molding process. The bridges comprise openings through which the flexible duplicate-molded material can penetrate and are designed to break while making the rigid plates separate from each other when the sole is first used, the bridges then being connected simply by the flexible plastic material.
This solution has proved to be costly, given the complexity of the mold made for that purpose and of the simultaneous-injection equipment used; furthermore, this solution offers no comfort layer.
What is sought is precisely the following:                obtaining greater walking comfort which is not neutralized by a stiffener inadequately arranged in the sole;        improving shock-absorption and ground-traction properties;        reaching a compromise between the lightest sole possible and a sole offering optimal ground traction, abrasion resistance, and torsional stiffness properties, while remaining relatively supple when flexed.        
This stiffness can not be achieved using soles made only of rubber, since they would have to be too thick and, consequently, too heavy. Moreover, it is difficult to control with precision the rigidity of a rubber sole, in particular in different directions.
In addition, for purposes of standardization and cost savings, a modularly-variable sole design is sought, which can be easily adapted at minimal cost to different uses and sports, i.e., mountain sports, golf, cycling, etc.